Surface followers and measuring apparatus of the general type to which this invention relates are described in the U.S. Pat. Nos. 1,946,924, Allen; 3,321,838, Albertson; 3,495,442, Rejsa; 3,528,002, Dunlavey; 3,855,524, Crawford, and 4,107,606, Typpo. Other background disclosures are contained in British Patent Specification No. 839,996 (1960); USSR Inventors' Certificate No. 589,545 (1978) and Japanese Patent Application Public Disclosure No. 52-38244 (1977).
Pneumatic surface followers are frequently used to maintain a predetermined distance between two objects. One or both objects may carry a transducer element to provide an input to a system for effecting measurement and control.
The thickness or caliper of moving sheets is commonly measured by passing the sheet over a roll or fixed plate on one side of the sheet while a surface follower automatically positions itself at a constant distance from the surface of the sheet on the other side. Either the plate or the surface follower or both may contain a proximeter element such as a magnetic reluctance or eddy current sensor element while the other may contain a target element. An unsupported sheet may travel between two surface followers, one of which may contain a proximeter while the other may contain a target element. The proximeter responds to its distance from the target, and the sheet thickness is derived in effect by subtraction of the constant distance or distances of the sheet follower or followers from the sheet surface or surfaces. It has been found, however, that errors arise when there are changes in the roughness of the sheet surface next to the pneumatic surface follower. The measurements have also been subject to substantial, erroneous variations caused by line speed changes, and hence expensive and time-consuming dynamic correlation procedures have been required for calibrating the instruments.